上期文章研究了资源加载流程，资源加载流程中，读取资源数据是在download过程中，如果我们要对资源进行解密，要改的就是这里了。

我做了一个加密插件：

image.png

支持对web平台和native平台进行加密，支持对文件名进行混淆。

image.png

c++端读取文件全都是通过FileUtils::getStringFromFile或者FileUtils::getDataFromFile进行操作，我们解密统一在这两个函数里进行就可以了。

image.png

对于web平台的解密，正常情况下，应该通过cc.assetManager.downloader.register注册不同的下载方法。但是为了不破坏源工程，我选择修改构建后导出的cocos2d-js.js文件，主要是修改有三点，解密代码放在downloadArrayBuffer，修改downloadText和downloadImage，改为通过downloadArrayBuffer先下载二进制流再解密解码为文本或图片。

var downloadArrayBuffer = function downloadArrayBuffer(url, options, onComplete) {
    options.responseType = "arraybuffer";
    url = _getRealPath(url)
    downloadFile(url, options, options.onFileProgress, function (err,data) {
        if(!err){
            ///解密
            data = hyz._decriptTool.decodeArrayBuffer(data);
        }
        onComplete&&onComplete(err,data)
    });
};
var downloadText = function downloadText (url, options, onComplete) {
    downloadArrayBuffer(url,options,function (err,data) {
    if(err){
        onComplete&&onComplete(err,data)
    }else{
        ///转化成Text
        hyz.arrayBuffer2Text(data,function(err,text) {
        if(err){
            onComplete&&onComplete(err,text)
        }else{
            onComplete&&onComplete(null,text)
        }
        })
    }
    })
};
var downloadJson = function downloadJson(url, options, onComplete) {
    downloadText(url,options,function (err,data) {
    if(err){
        onComplete&&onComplete(err,data)
        return;
    }
    if (!err && typeof data === 'string') {
        try {
            data = JSON.parse(data);
        }
        catch (e) {
            err = e;
        }
    }
    onComplete && onComplete(err, data);
    })
};
var downloadImage = function downloadImage(url, options, onComplete) {
    downloadArrayBuffer(url,options,function(err, data){
    if(err){
        onComplete&&onComplete(null,data);
        return;
    }
    let index = url.lastIndexOf(".");
    let suffix = url.substr(index+1);
    let typeStr = hyz.imgTypes[suffix]||hyz.imgTypes["png"]

    if(cc.sys.capabilities.imageBitmap){
        let blob = new Blob([data],{type:typeStr})
        onComplete&&onComplete(null,blob);
        cc.log(blob)
    }else{
        let base64code = hyz.arrayBufferToBase64Img(data);
        base64code = `data:${typeStr};base64,${base64code}`
        downloadDomImage(base64code,options,onComplete)
    }
    })
};

如上，主要是downloadArrayBuffer进行解密，另一个要注意的就是下载图片要手动转化一下。

加密解密使用的最简单的异或加密，代码如下：

encodeArrayBuffer(arrbuf,sign=this.encriptSign,key=this.encriptKey) {
        if(this.checkIsEncripted(arrbuf,sign)){
            return
        }
        let signBuf = new Uint8Array(this.strToBytes(sign));
        let keyBytes = this.strToBytes(key)
        let buffer = new Uint8Array(arrbuf);
        
        let _outArrBuf = new ArrayBuffer(signBuf.length+buffer.length)
        let outBuffer = new Uint8Array(_outArrBuf)
        for(let i=0;i<signBuf.length;i++){
            outBuffer[i] = signBuf[i]
        }
        let idx = 0;
    
        for(let i=0;i<buffer.length;i++){
            let b = buffer[i];
            let eb = b^keyBytes[idx]
            if(++idx>=keyBytes.length){
                idx = 0
            }
            outBuffer[signBuf.length+i] = eb
        }
        
        return outBuffer;
    }
    
    decodeArrayBuffer(arrbuf,sign=this.encriptSign,key=this.encriptKey){
        if(!this.checkIsEncripted(arrbuf,sign)){
            return arrbuf;
        }
        let signBuf = new Uint8Array(this.strToBytes(sign));
        let keyBytes = this.strToBytes(key);
        let buffer = new Uint8Array(arrbuf);
    
        let size = buffer.length-signBuf.length;
        let _outArrBuf = new ArrayBuffer(size)
        let outBuffer = new Uint8Array(_outArrBuf)
        let idx = 0;
        for(let i=0;i<size;i++){
            let b = buffer[signBuf.length+i];
            let db = b^keyBytes[idx]
            if(++idx>=keyBytes.length){
                idx = 0
            }
            outBuffer[i] = db;
        }
    
        return outBuffer;
    }

至于文件名混淆，就是对源uuid格式的文件名和一个混淆签名再次进行md5，得到新文件名。
如aaaa-bbbb-cccc-dddd.png变为1111-2222-3333-4444.png
关键是工程中如何识别新文件名。项目运行时，查找文件是根据文件路径查到uuid再组装成url的，我们只需要在url组装后，再按照我们的混淆规则，再算一次就好了，保险起见，只混淆uuid格式的文件名。native模式下，修改jsb-engine.js的transformUrl，对本地url进行再次转换；web模式下，downloadArrayBuffer里直接在下载之前将url转换成混淆后的文件名。

function _getRealPath(path) {
    let excludeChangeNameList = [".mp3",".ogg",".wav",".js",".jsc",]
    if(path.indexOf("assets")!=0){
        return path
    }
  if(!true){//tag
    return path;
  }
    for(let ext of excludeChangeNameList){
      if(path.endsWith(ext)){
          return path
      }
    }
    var ext = path.substr(path.lastIndexOf("."));
    var arr = path.split('/');
    let name = arr[arr.length-1];
    let realPath = path;

    if(name[8]=="-"&&name[13]=="-"&&name[18]=="-"&&name[23]=="-"){
        let md5 = hyz.str_to_md5(name+"ggg5675")
        let arr2 = [8,13,18,23]
        for(let i = arr2.length-1;i>=0;i--){
          let idx = arr2[i];
          md5 = md5.slice(0, idx) + "-" + md5.slice(idx);
        }
        md5+=ext;
    
      realPath = path.replace(name,md5);
      realPath = realPath.replace("/"+name.slice(0,2)+"/","/"+md5.slice(0,2)+"/");
      realPath = realPath.replace("\\"+name.slice(0,2)+"\\","\\"+md5.slice(0,2)+"\\");
    }

    return realPath
};

完整插件源码在GitHub